Nalmefene is a known opioid receptor antagonist which can inhibit pharmacological effects of both administered opioid agonists and endogenous agonists deriving from the opioid system. The clinical usefulness of nalmefene as antagonist comes from its ability to promptly (and selectively) reverse the effects of these opioid agonists, including the frequently observed depressions in the central nervous system and the respiratory system.
Nalmefene has primarily been developed as the hydrochloride salt for use in the management of alcohol dependency, where it has shown good effect in doses of 10 to 40 mg taken when the patient experiences a craving for alcohol (Karhuvaara et al., Alcohol. Clin. Exp. Res., (2007), Vol. 31 No. 7. pp 1179-1187). Additionally, nalmefene has also been investigated for the treatment of other addictions such as pathological gambling and addiction to shopping. In testing the drug in these developmental programs, nalmefene has been used, for example, in the form of parental solution (Revex™).
Nalmefene is an opiate derivative quite similar in structure to the opiate antagonist naltrexone. Advantages of nalmefene compared to naltrexone include longer half-life, greater oral bioavailability and no observed dose-dependent liver toxicity.
Nalmefene differs structurally from naltrexone in that the ketone group at the 6-position of naltrexone is replaced by a methylene (CH2) group, which considerably increases binding affinity to the μ-opioid receptor. Nalmefene also has high affinity for the other opioid receptors (κ and δ receptors) and is known as a “universal antagonist” as a result of its ability to block all three receptor types.
Nalmefene can be produced from naltrexone by the Wittig reaction. The Wittig reaction is a well known method within the art for the synthetic preparation of olefins (Georg Wittig, Ulrich Schöllkopf (1954). “Über Triphenyl-phosphin-methylene als olefinbildende Reagenzien I”. Chemische Berichte 87: 1318), and has been widely used in organic synthesis.
The procedure in the Wittig reaction can be divided into two steps. In the first step, a phosphorus ylide is prepared by treating a suitable phosphonium salt with a base. In the second step the ylide is reacted with a substrate containing a carbonyl group to give the desired alkene.
The preparation of nalmefene by the Wittig reaction has previously been disclosed by Hahn and Fishman (J. Med. Chem. 1975, 18, 259-262). In their method, naltrexone is reacted with the ylide methylene triphenylphosphorane, which is prepared by treating methyl triphenylphosphonium bromide with sodium hydride (NaH) in DMSO. An excess of about 60 equivalents of the ylide is employed in the preparation of nalmefene by this procedure.
For industrial application purposes, the method disclosed by Hahn and Fishman has the disadvantage of using a large excess of ylide, such that very large amounts phosphorus by-products have to be removed before nalmefene can be obtained in pure form. Furthermore, the NaH used to prepare the ylide is difficult to handle on an industrial scale as it is highly flammable. The use of NaH in DMSO is also well known by the skilled person to give rise to unwanted runaway reactions. The Wittig reaction procedure described by Hahn and Fishman gives nalmefene in the form of the free base. The free base is finally isolated by chromatography, which may be not ideal for industrial applications.
U.S. Pat. No. 4,535,157 also describes the preparation of nalmefene by use of the Wittig reaction. In the method disclosed therein the preparation of the ylide methylene triphenylphosphorane is carried out by using tetrahydrofuran (THF) as solvent and potassium tert-butoxide (KO-t-Bu) as base. About 3 equivalents of the ylide are employed in the described procedure.
Although the procedure disclosed in U.S. Pat. No. 4,535,157 avoids the use of NaH and a large amount of ylide, the method still has some drawbacks which limit its applicability on an industrial scale. In particular, the use of THF as solvent in a Wittig reaction is disadvantageous because of the water miscibility of THF. During the aqueous work-up much of the end product (nalmefene) may be lost in the aqueous phases unless multiple re-extractions are performed with a solvent which is not miscible with water.
Furthermore, in the method described in U.S. Pat. No. 4,535,157, multiple purification steps are carried out in order to remove phosphine oxide by-products of the Wittig reaction. These purification steps require huge amounts of solvents, which is both uneconomical and labor extensive requiring when running the reaction on an industrial scale. As in the case of the Wittig reaction procedure described by Hahn and Fishman (see above) the Wittig reaction procedure disclosed in U.S. Pat. No. 4,535,157 also yields nalmefene as the free base, such that an additional step is required to prepare the final pharmaceutical salt form, i.e. the hydrochloride, from the isolated nalmefene base.
U.S. Pat. No. 4,751,307 also describes the preparation of nalmefene by use of the Wittig reaction. Disclosed is a method wherein the synthesis is performed using anisole (methoxybenzene) as solvent and KO-t-Bu as base. About 4 equivalents of the ylide methylene triphenylphosphorane were employed in this reaction. The product was isolated by extraction in water at acidic pHs and then precipitating at basic pHs giving nalmefene as base.
Even though the isolation procedure for nalmefene as free base is simplified, it still has some disadvantages. The inventors of the present invention repeated the method disclosed in U.S. Pat. No. 4,751,307 and found that the removal of phosphine oxide by-products was not efficient. These impurities co-precipitate with the nalmefene during basification, yielding a product still contaminated with phosphorus by-products and having, as a consequence, a low chemical purity, as illustrated in example 2 herein.
There is therefore a need within the field to improve the method of producing nalmefene by the Wittig reaction. In particular, there is a need for a method that is readily applicable on a large industrial scale and which avoids the use of water-miscible solvents, such as THF, in the Wittig reaction, and permits easy isolation of nalmefene in a pure form suitable for its transformation to the final pharmaceutical salt form.
The inventors of the present invention have met this goal by developing an improved method for the preparation of nalmefene which is applicable on an industrial scale and results in a highly pure nalmefene as the hydrochloride salt.